The cumulative probability to have an IGM attenuation in NB359 for z = 3.09 sources smaller than that in the horizontal axis, based on our Monte Carlo simulations: the dashed line (Inoue & Iwata 2008) and the solid line (updated version). The number of realizations of lines of sight is 10 000. The narrow-band filter NB359 captures 852–905 Å in the rest frame of the sources. The inset shows a close-up of the smallest attenuation.